1. Field of the Invention
The present invention relates to a process and system for removing a desired fraction from a liquid using a centrifugal extractor.
2. Description of Related Art
Extraction processes using dense phase (i.e., a supercritical fluid or a liquefied gas) solvents have been in commercial operation for 50 years. Extraction from solid materials has been done in a batch manner, periodically stopping the extraction process to depressurize, unload, reload, and repressurize the extraction vessel prior to resuming extraction. Multiple batch extraction vessels in series, with the periodic removal of a single vessel from the extraction train, have reduced but not eliminated the lost processing time. For example, U.S. Pat. Nos. 5,288,511 and 5,312,635 to Kazlas et al. disclose using supercritical carbon dioxide in batch processes to decaffeinate acidified coffee and to remove oil from fried food products, respectively.
The processing equipment used in continuous extractions consists of vertical cylindrical columns similar to that used for low-pressure distillation or liquid-liquid extraction. This equipment operates in either a batch or continuous manner to extract components from a variety of sources.
Prior art solutions to the problem of increasing production rates in the extraction of liquids with dense phase solvents have been limited to the application of various distributors in vertical cylindrical columns. More recent attempts have been described in U.S. Pat. No. 4,956,052 to Hirata et al. As in liquid-liquid extraction, the use of different distributors reduces the vertical circulation of the continuous phase and promotes high interfacial contact between the dispersed and continuous phases. All of these solutions, however, are limited by the force of gravity to effect the counterflow of liquids of different density.
Notwithstanding prior art solutions, a problem associated with the commercial utilization of dense phase solvent extraction processes continues to be the high processing cost, relative to alternate extraction methods. This high processing cost, due principally to limited production rates in capital intensive process equipment, has limited the application of dense phase extraction processes to high-value products.
These products are typically produced in small volumes and by a small number of processors, inherently limiting the wide-scale utility of this technology. An extraction process that results in reduced processing costs due to increased production rates would facilitate extending the utilization of this technology to a greater range of products.
Centrifugal extractors, such as Podbielniak and Westfalia extractors, increase production capacity in liquid-liquid extraction systems and overcome the limitations of gravity as the driving force for separation. For example, U.S. Pat. No. 2,044,996 to Podbielniak, combines the advantages of multi-stage countercurrent contact of two fluid phases with the ability of a centrifuge to process large volumes of material quickly in a small compact device. Similarly, Wem, in U.S. Pat. No. 4,382,900, discloses a centrifugal gas-liquid apparatus that relies upon an annular packing of gas/liquid permeable material in the extractor. The extractor rotates to force liquid radially outward through the permeable material with centrifugal force where it encounters a gas before exiting through a separate chamber. Similarly, the gas passes through the liquid and permeable material and exits through a gas outlet. These centrifugal extractors only operate with liquid-liquid or gasliquid systems.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide an extraction process and apparatus that result in reduced processing costs due to increased production rates.
It is another object of the present invention to provide a process and apparatus that will permit the continuous processing of large volumes of liquids with dense phase fluids in a smaller, more compact device.
Still other objects and advantages of the invention will be apparent from the specification.